1. Field of the Invention
The present invention relates to a stepping motor servo control system for a floppy disk drive (hereinafter called "FDD"), and more particularly, to a stepping motor servo control system which is operated by receiving feedback information from erase heads based on a closed loop servo system.
2. Description of the Related Art
FDDs have progressed rapidly from the beginning of the 1970's to the middle of the 1980's. Namely, the size of the disk has been reduced from 8 inches to 3.5 inches whereas the data recording density has increased by switching from FM to MFM (Modified Frequency Modulation). Also, single-sided disks have been replaced with double-sided disks, the track density (tracks per inch: TPI) and bits per inch (BPI) have been increased, and nowadays the recording capacity of 2M bytes with 3.5-inched disks is achieved as in an open loop servo system. In order to increase the recording density more, the track density should be increased.
Though maximum values of 96 TPI in case of 5.25-inched disk and 135 TPI in case of 3.5-inched disk with the open loop servo system are now used practically, it is impossible to increase beyond these values due to the limitation of tolerance of drive mechanism and media with respect to changes in temperature and humidity. In order to achieve the track servo function for determining the head position, all standard type FDDs have employed an open loop servo motor in view of the problems of cost and interchangeability of media.
With the closed loop servo system, it is posible to increase the track density beyond 1000 TPI as in case of Winchester disk drive (hereinafter called "WDD"), and 5-10M bytes of high capacity drive has appeared with the level of 300 TPI in the case of FDD. However, unlike the WDD, special media and accessary components are required to produce the FDD of high track density on a commercial scale. In other words, even in the case that the medium is a flexible disk, a special plastic hard case in the shape of a cartridge is necessary, and a separate servo track with information for determining the head position needs to be included when the recording medium is manufactured. Therefore, using an FDD utilizing the closed loop servo system was not apparent due to the problems of cost and the interchangeability of the recording media.
At present, dedicated servo system, embedded servo system, modified dedicated servo system, etc. are mostly used in the field of the WDD as the closed loop servo system. For all of them, the servo track information should be prerecorded on the tracks for determining a head position. Therefore, two different burst signals are recorded on both sides of the center of the track as track servo information and the determinaton of the head position is controlled by reading these burst signals. Referring to FIG. 1A, a head (not illustrated) reads burst signals 15A and 15B recorded at both sides of the center 13 of a certain track and by comparing the read burst signal levels 16A and 16B as shown in FIG. 1B, a servo motor (not illustrated) is guided to the position where the signal levels 16A and 16B become identical. Therefore, the burst signals 15A and 15B which are repeatedly recorded on the data track are continuously read out until the read levels will become identical.